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Parallel wiring in the
circuit breaker panel!
No matter which path the river takes, its
change in elevation, from the mountaintop to
the plain, is the same.
Some rivers flow downhill in a single stream.
Other rivers may split into two or more
smaller streams as they flow over a waterfall
or through a series of rapids. In this case,
part of the river follows one path, while
other parts of the river follow different
paths.
No matter how many paths the river takes,
however, the total amount of water flowing
down the mountain remains unchanged. In
other words, the amount of water flowing
downhill is not affected by the path it takes.
How does the river model an electric circuit?
The distance that the river drops is similar to the potential
difference in a circuit.
The amount of water flowing in the river is similar to current
in a circuit.
Narrow rapids create resistance and
are similar to resistors in a circuit.
The SUN provides the “energy” for
the circuit through the Hydrologic
Cycle., like a battery or generator
provides for a circuit.
Recall from Chapter 20 that charge cannot be created or
destroyed. Because the charge in the circuit has only one path
to follow and cannot be destroyed, the same amount of charge
must leave a circuit as enters the circuit.
This means that the current is the same everywhere in the
circuit. If you connect three ammeters in the circuit, as shown,
they all will show the same current. A circuit such as this, in
which all current travels through each device, is called a
series circuit.
Ohm’s Law written for
this circuit
Notice that the equivalent resistance is greater than that of
any individual resistor. Therefore, if the battery voltage does
not change, adding more devices in series always decreases the
current
Voltage drops in a series circuit
As current moves through any circuit, the net
change in potential must be zero. This is
because the circuit’s electric energy source,
the battery or generator, raises the potential
an amount equal to the potential drop
produced when the current passes through the
resistors. Therefore, the net change is zero
All of the potential (Voltage!) is “lost” as the current moves
through the circuit. It is not really lost, but converted to
other forms of energy!
An important application of series
resistors is a circuit called a
voltage divider. A voltage divider
is a series circuit used to produce
a voltage source of desired
magnitude from a higher-voltage
battery. For example, suppose you
have a 9-V battery but need a 5-V
potential source.
You can choose RA so that it “uses
up” 4V of the Potential, leaving 5V
across RB
The resistance of a photoresistor
depends upon the amount of light that
strikes it. Photoresistors are made of
semiconductors, such as silicon, selenium,
or cadmium sulfide.
A typical photoresistor can have a
resistance of 400 Ω when light is striking
it compared with a resistance of 400,000
Ω when the photoresistor is in the dark.
The voltage output of a voltage divider
that uses a photoresistor depends upon
the amount of light striking the
photoresistor sensor.
This circuit can be used as a light meter
in a camera or VCR camera.
PARALLEL
CIRCUIT
How many current paths are there? The current from the generator
can go through any of the three resistors. A circuit in which there are
several current paths is called a parallel circuit. The three resistors
are connected in parallel; both ends of the three paths are connected
together.
In the mountain river model, such a circuit is illustrated by three
paths for the water over a waterfall. Some paths might have a large
flow of water, while others might have a small flow. The sum of the
flows, however, is equal to the total flow of water over the falls. In
addition, regardless of which channel the water flows through, the
drop in height is the same.
In a parallel circuit, the total
current is equal to the sum of
the currents in the
individual paths.
This one is a bit harder to figure out for the
“Effective Resistance”
A short circuit occurs when a circuit with a very low resistance is
formed. The low resistance causes the current to be very large.
When appliances are connected in parallel, each additional appliance
placed in operation reduces the equivalent resistance in the circuit
and increases the current through the wires. This additional current
might produce enough thermal energy to melt the wiring’s insulation,
cause a short circuit, or even begin a fire.
A fuse is a short piece of metal that melts when too large a current
passes through it. The thickness of the metal used in the fuse is
determined by the amount of current that the circuit is designed to
handle safely. If a large, unsafe current passes through the circuit,
the fuse melts and breaks the circuit.
A circuit breaker, is an automatic switch that opens when the
current reaches a threshold value. If there is a current greater than
the rated (threshold) value in the circuit, the circuit becomes
overloaded. The circuit breaker opens and stops the current.
When too much current flows through the
bimetallic strip, the heat that is generated causes
the strip to bend and release the latch. The handle
moves to the off position, causing the switch to
open and break the circuit
Car
Amplifier
Old home
fusebox
The current in any one circuit does not depend
upon the current in the other circuits.
Suppose that a 240-W television is plugged
into a 120-V outlet. The current is represented
by I = P/V.For the television, I = (240 W)/(120
V) = 2.0 A.
When a 720-W curling iron is plugged into the
outlet, its current draw is I = (720 W)/(120 V)
= 6.0 A.
Finally, a 1440-W hair dryer is plugged into the
same outlet. The current through the hair
dryer is I = (1440 W)/(120 V) = 12 A.
The resistance of each appliance can be
calculated using the equation R = V/. The
equivalent resistance of the three appliances is
as follows.
A fuse is connected in series with the power
source so that the entire current passes through
it. The current through the fuse is calculated
using the equivalent resistance.
If the fuse in the circuit is rated as 15 A,
the 20-A current would exceed the rating
and cause the fuse to melt, or “blow,” and
cut off current.
A “short circuit” could occur if the insulation on a
lamp cord became old and brittle. The two wires in
the cord might accidentally touch, resulting in a
resistance in the wire of only 0.010 Ω. This
resistance results in a huge current.
Find Equivalent resistance of
Parallel resistors. Show this
equivalent resistance as a
“new” resistor in series with
the others.
Add together the resistances
of all series resistors
Show circuit with just one
“equivalent” resistor!
An ammeter is a device that is
used to measure the current in
any branch or part of a circuit.
If, for example, you wanted to
measure the current through a
resistor, you would place an
ammeter in series with the
resistor. This would require
opening the current path and
inserting an ammeter. Ideally,
the use of an ammeter should not
change the current in the circuit.
A voltmeter, is used to measure
the voltage drop across a portion
of a circuit. To measure the
potential drop across a resistor, a
voltmeter is connected in parallel
with the resistor. Voltmeters are
designed to have a very high
resistance so as to cause the
smallest possible change in
currents and voltages in the
circuit.
10,000 Ω
How does a Ground
Fault Interrupter
(GFI) Work?
The End!